Field of the Invention
The present invention is directed generally to implantable heart pacemaking systems, and more specifically to a heart pacing system having an improved capability to sense cardiac response, with the system being able to verify capture even when pacing in a bipolar configuration.
"Capture" is defined as a cardiac response which is initiated by a pacemaker stimulation pulse. When a pacemaker stimulation pulse stimulates either the atrium or the ventricle of the heart during an appropriate portion of a cardiac cycle, it is desirable that the stimulation pulse be sufficient to cause the heart to respond to the stimulation pulse provided. Every patient has a threshold which is generally defined as the minimum amount of stimulation energy required to effect capture. It is usually desired to achieve capture at or near the lowest possible energy setting to conserve battery power, thereby extending the useful life of the pacemaker. Typically, once the lowest setting at which capture is reliably obtained is determined, the pulse amplitude is increased somewhat to provide a safety margin so that if the patient's threshold increases, the output of an implanted pacemaker will still be sufficient to maintain capture.
Capture is typically assessed by means of an electrocardiogram (ECG) measured through ECG electrodes placed on the patient's limbs and/or chest. When a patient is connected to a typical ECG monitor and the pacemaker is providing stimulation pulses, the physician monitors the output to assess whether each pacing pulse, which is seen as a spike, is followed by a cardiac response. Ventricular capture is relatively easy to assess in that each ventricular stimulation pulse which achieves capture produces a very large R-wave.
Determination of atrial capture in response to an atrial stimulation pulse is a more difficult task. Atrial capture in response to an atrial stimulation pulses has been viewed on an electrocardiogram as a P-wave following the atrial stimulation pulse by a constant time interval. One previous device utilized dual sensing electrodes and suggested that the heart action is 15 to 20 milliseconds after the stimulus. (See U.S. Pat. No. 4,365,639, to Glodreyer.)
However, the time delay varies considerably depending on the patient's physiology, administered drugs, electrolyte balance, proximity of the sensing electrode to the stimulating electrode, and other factors. Further, it is almost impossible to guarantee that a P-wave will be of sufficient amplitude to be seen on a standard ECG scan. In order to verify atrial capture in patients with intact cardiac condition, the physician must generally pace atrially and observe ventricular response to the paced atrial rate. However in patients with heart block, the physician may not be able to determine atrial capture because of the lack of conduction from the atrium to the ventricle, thus preventing the ventricle from responding to atrial stimulation pulses. In such cases the physician may have to rely on fluoroscope evaluation of cardiac wall motion in response to the atrial stimulation to ascertain atrial capture.
Another method for determining atrial capture is to transmit the signal appearing on the atrial stimulation electrode to an external viewing device. Some of the newer pacemakers have the capability to transmit intracardiac electrogram (IEGM) signals appearing at either the atrial electrode or the ventricular electrode in real time to an external monitoring device for real-time evaluation by a physician. (See, for example, U.S. Pat. No. 4,232,679, to Schulman.)
However, due to the large magnitude of a stimulation pulse with respect to the P-wave signal, and the closeness in time between the stimulation pulse and the occurrence of the P-wave, the atrial sensing amplifiers of conventional pacemakers tend to saturate in the presence of a stimulation pulse and mask the P-wave. Thus, as a practical matter, utilization of IEGM signals derived from the stimulation electrode is not effective for determining if P-wave capture has occurred.
Pacing systems for verifying capture are disclosed in U.S. Pat. Nos. 4,686,988 and 4,817,605, both to the present inventor. In the systems taught in those patents, as in all known systems for verifying capture, one of the two electrodes used to sense capture must be indifferent. This requirement is due to the fact that the two electrodes which are used for pacing will have a charge which inhibits their ability, as a pair, to detect capture.
The systems taught by these two patents use bipolar leads pacing in a unipolar configuration. Thus, only one of the electrodes on each lead is used to pace, with the case of the pacemaker acting as the other pacing electrode. The second electrode on the lead is utilized as an indifferent electrode together with the pacemaker case to sense capture. Thus, for example, in the systems described in U.S. Pat. Nos. 4,686,988 and 4,817,605, the pacing configuration used a unipolar configuration, tip to case. Capture is detected using the ring electrode and the case. U.S. Pat. Nos. 4,686,988 and 4,817,605 are both hereby incorporated herein by reference.
This technique has two significant disadvantages. The first disadvantage is that even with a bipolar pacemaker and bipolar leads, capture can only be sensed when pacing is undertaken in a unipolar configuration. Thus, bipolar pacing and capture detection cannot take place together. The second disadvantage is that the system of U.S. Pat. Nos. 4,686,988 and 4,817,605 cannot be used with unipolar leads.
It is accordingly the primary objective of the present invention that it provide a system which is capable of reliably sensing capture, and that the system be capable of sensing capture on a continual basis. It is a further objective that the system of the present invention be capable of sensing capture while pacing in a bipolar configuration. It is also an objective of the present invention that it provide a system for reliably sensing capture for use with a unipolar pacing system having unipolar leads.
The improved system of the present invention should be relatively simple to implement, so that it will not require an increase in the size of the pacemaker. In addition, it should simultaneously significantly extend the operating life of the pacer, and constantly maintain capture, thus benefiting the patient in two important ways. Finally, it is an objective that all of the aforesaid advantages and objectives be achieved without incurring any substantial relative disadvantage.